Density is the thing though isn't it. If we are talking about maximum weight savings then the GG pads still win, 82g for 1500x500x3mm. Logically, it seems that less dense EVA should be warmer too since it would contain more air, but I'm sure it's more complicated than that.

What's the difference between their EVA30 and the EVA75? And is that stuff Evazote? The black EVA75 looks like it could be...

Yes I suppose the difference in R values is not huge, but if I don't get it I'll always want it ;). I might see what this local guy has got in the 3-4mm range and see how that performs as it gets colder.

It definitely seems to be the case that the extra insulation added by thin CCF layers is not as simple at simply adding R-values, so a +/- 10% difference in insulation value probably isn't going to make a noticeable difference, all other factors being equal.

I have not used or even handled the syn mat but, based on my experience with the UL7 down mat, I am surprised by peoples' need to use the syn mat with a foam pad to ensure warmth. Based on my experience I would have thought the R value difference between syn and down to be less obvious at those temperatures, i.e above freezing.

These are anecdotal examples but illustrative of the effective R value of the UL7 down mat for me:

1. I have slept in -24C (-11F) on a UL7 LW downmat, inside a 3 season tent (BS Evolution, with only the sylnylon floor between the pad and snow, ie. no foam pad. I wore 2 merino base layers (150 grams each), a toque and fleece neck warmer. I was in a WM Ultralite bag (rated to -7C\20F) covered with EE Revelation 20F (rated -7C\20F), cinched tight and I was toasty warm.

2. Last year while sleeping on a rocky river bed (rounded boulders cemented in place by compact sand) my down mat kept deflating and after pumping it up twice early in the night, and then assuming I had a leak, I managed to find a bearable sleep position and left it deflated and slept on it without air for about 6 hours. The temperature that night got down to -1C (30F). Again, I was toasty warm. Used same bag as above but no quilt.

The next night, the last night of the trip, assuming it had a puncture and because I was sleeping on a sandy beach which was acceptably comfortable, I didn't use my mat and slept with only my sleeping bag (same bag as above) and the silnylon tent floor between me and the ground. That night the temperature went down to 1C (34F) and I was cold the whole damn night, to such a degree that I mummified the bag to the fullest extent possible and maintained a fetal position to stay warm.

Conclusion, the down in the deflated mat provided considerable insulation even without air in the chambers.

Maybe the down mats are a whole lot warmer than the syn mats. I am unsure how R values work. Perhaps the increase in value has exponential effect.

I have both and experienced feeling a bit cool on the syn mat. I found myself augmenting the mat with a 1/4" ccf pad on most trips and ultimately added the UL downmat.

Down to about freezing I found the syn mat to be adequate if a little cool, but not so much that it interrupted my sleep. With the UL downmat I am toasty but haven't been below 0 f and it is lighter than the ccf and ULSyn combo.

I can't remember the last time I camped out that the night temps were warmer than 50f or so, and as a result I find that the ul downmat isn't too warm and has allowed me to take a lighter bag.

I tried the x therm and the all season but didn't like how narrow they felt but both were plenty warm and packed down smaller than the downmat. If the x therm was a bit wider and not mummy shaped I would most likely have kept it.

"Maybe the down mats are a whole lot warmer than the syn mats. I am unsure how R values work. Perhaps the increase in value has exponential effect."

The downmats are significantly warmer than the synmats, just as a down jacket of a given fill weight is warmer than a synthetic insulated jacket of the same fill weight. As far adding R values goes they do stack linearly, and usable temperature vs. R value also seems to be a linear relationship. I.e. you need roughly the same amount of extra R value to get from 0C to -10C as you do to get from -10C to -20C, etc.

I went to a craft shop and bought two small sheets of EVA: 2mm x 300mm x 455m (A3; $3) and 3mm x 300mm x 225mm (A4; $2.50). On my scales the A3 sheet weighs 27g, and the A4 sheet 14.5g. According to my calculations that makes the density correspond to something like EVA105 and EVA75, respectively (see for e.g., http://metrofoam.com.au/eva-foam.html).

That's a lot more dense than the EVA30 I think Gossamer Gear is using in their Thinlight pad, but at a couple of bucks I think it's worth a try.

I actually bought two of the A4 sheets, so I'm planning to tape them together and try my mat again by itself, supplemented by the 2mm foam, and supplemented by the 3mm foam.

Any ideas on how to keep the piece of foam centered beneath my hips? Just put it on top of the inflatable pad?

Gossamer Gear's thin light stuff seems to be between 35 and 40 kg/m^3. My local guy (ACT) seems to be able to get EVA in various densities from 30 to 75, in almost any thickness. A two by one by 4mm sheet will cost me 50 bucks. Seems like a good option.

The question is now whether to get EVA30 or EVA45. Thermal conductivity vs. weight vs. density. A denser mat will compress less but a less dense mat has sightly lower thermal conductivity and is lighter.

trying to sort out the convective heat loss is coming from the air in the mattress so the ambient air as well as the ground - seems like on top makes the most sense from that perspective but I could rather easily have the theory wrong.

Having said that I have the heavier Synmat 7 (not UL) and it has a good bit higher r rating of 4.9. I was cozy in 22 degrees with my 20 degree down bag and medium weight base layers - a 100g polartec pullover and beanie (I'm bald so a beanie is essential, well, most of the time other than late spring/summer).

Source of foam and extra foam on top or underneath mat?
on 05/23/2013 18:31:43 MDT

@ Jeremy - This company quoted me only $35 including postage for a 2m x 1m x 3mm sheet of EVA30: http://www.kangarubber.com.au/eva.html.

@ Kate - thanks for that.

@ Phillip - I think that's a great question. If it was much of a muchness, then as Kate suggested having the extra foam underneath with some silicon on both sides would seem to have the added benefit of stopping everything moving around.

Re: Source of foam and extra foam on top or underneath mat?
on 05/24/2013 00:39:30 MDT

That's cheap, but only about 10 dollar less than the local one. Also this stuff is black rather than blue... dunno if that means there's a difference in quality. Apparently the zotefoams blue foams are less desirable than the evazote black stuff.

As far as under or over the mat. I have heard about people putting the CCF mat over their inflatable mat but that makes no sense to me. It should make at most no difference, I can't see why it would be warmer.

EDIT: Sorry! Got it totally backwards. Meant to say it makes no sense to put the CCF OVER the inflatable mat!

"As far as under or over the mat. I have heard about people putting the CCF mat under their inflatable mat but that makes no sense to me. It should make at most no difference, I can't see why it would be warmer."

CCF under is useful for added protection from ground. CCF over has been empirically tested to be warmer. The explanation isn't known but the working theory is that airmats lose more heat to convection and radiation from the tall sides than a CCF. A ccf then provides an initial barrier that reduces the amount of heat lost by these factors. When below you then you have a greater differential in temps within the CCF which increases convective forces along with a greater temp differential along the sides which increases radiative losses. If sleeping pads only lost heat through the vertical axis and conduction then they would be simply additive and it would not matter the orientation. The sides and convection within an air mat do affect the insulation such that order has been shown to matter (to a small degree).

Model it this way, your body is a point source for heat and that heat is lost to exposed surface area of the sleeping pad (like arcs across a semi circle). The air mat has a greater surface area than a ccf. So to maximize insulation (or rather minimize heat loss) you want the item with the least surface area closest to your body. If you did it the other way around, you'd need more CCF to cover all the exposed sides of an air mat (so not just the bottom, but also the sides, like a bathtub) to have equivalent total insulation...which of course is heavier and less efficient.

There's also some potential benefit by having an extra layer of trapped air close to the heat source rather than the heat sink and to also distribute weight more evenly across the top surface minimizing cold spots due to pressure points.

Is any of this particularly noticeable to the sleeper? YMMV... Some even claim that adding a radiative barrier (like foil laminated to bubblewrap products) INSIDE your sleeping bag is warmer than having it between a bag and on top of an air mat, or under the airmat. Similar rationale as above applies with the nuance that reflective barriers work best the closer they are to the initial heat source because there is less matter to transport heat away before the reflective barrier can do it's best work.

"There's also some potential benefit by having an extra layer of trapped air close to the heat source rather than the heat sink and to also distribute weight more evenly across the top surface minimizing cold spots due to pressure points."

Sort of makes sense if I think of in what order I wear my thermal top, shirt, jumper and rain jacket. I wear it in that order, and that order is warmer than if I did it in any other order. Having the tight fitting thermal next to skin creates a uniform layer of trapped air that's less susceptible to being disturbed than, say, the shirt.

"and to also distribute weight more evenly across the top surface minimizing cold spots due to pressure points"

Doesn't make sense re the Exped mats since the mats would never compress enough to compress the insulation. But your theory about the sides of the mat makes sense. That suggests that letting a quilt drape over the sides of the mat would help increase warmth too.